task was a mix of both conditions. Only a single icon was highlighted and needed to be verified, but an additional two icons moved. The purpose of the distractor task was to pro- vide insight about how motion on the screen affects user performance with animation and phosphor.

a

b

Figure 16: The tripleIcon task. (a) Three icons are highlighted in green and a target window in red, then (b) three icons perform a transition, here shown using the phosphor interface condition. In this example the transition matches the earlier high- lighting, so the correct response is to press “Y”.

Interfaces We tested two interface conditions. The phosphor interface (Figure 16) used the speed line design described earlier and the copy visuals shown in Figure 5. Phosphor cues were shown the moment the transition was triggered and were removed when the participant hit the answer button. The animation interface used slow-in/slow-out animation at 25 frames per second. If multiple icons moved, they moved in synchrony. There were five animation conditions with animation durations of 125ms, 250ms, 500ms, 1000ms, and 2000ms, covering the combined range from several experi- ments reported in the related work, e.g. [19, 28].

Experimental design The study was a 6 × (Interface: phosphor, 5 × animation) × 3 (Task: SingleIcon, TripleIcon, Distractor) × 4 (Outcome: Correct, ErrorUndershoot, ErrorNeighbor, ErrorOther) within-subjects design. During the study, participants per- formed each task in its entirety using all six interfaces be- fore moving to the next task. Interface and task order were counterbalanced. Participants performed the same 60 trials (15 icon layouts x 4 outcomes) for each of the six interfaces of a given task in different randomized orders. This allowed us to control for the difficulty of the trial. Pilot testing and analysis on the study data showed no learning effects. For each trial, we recorded task completion time and error. Task time was counted from the moment the transitions started until the moment the participant pressed one of the answer keys.

Participants received training at the beginning of the ex- periment and prior to starting each block. At the end of the study, they filled in a questionnaire. Overall, the study took about 90 min per participant.

Apparatus The experiment was run on three PCs running WindowsXP with LCD screens driven by NVIDIA graphics cards and offering a 60Hz refresh rate. The test program was 1024x768 large (11”/28cm wide). Participants interacted with the system using a standard PC keyboard.

Participants Twelve volunteers (4 female) between the ages of 22 and 35 participated in this study. All had experience with graphical user interfaces and had normal or corrected to normal vision and color vision. Each received a small gratu- ity for their time.

Hypotheses Our main hypothesis was that the phosphor interface, which allows users to view transitions at their own pace, would perform as well as the best animation condition. We also assumed that the harder tasks would impact task perform- ance across interface conditions, but we had no clear hy- pothesis about how.

Results We analyzed the performance data at the summary level, averaging the 15 trials within each condition. We used a 6 (Interface) × 3 (Task) × 4 (Outcome) repeated measures analysis of variance (RM-ANOVA) for each of the depend- ent measures, trial time and error rate.

Task time phosphor vs. animation: For average trial time, we observed a main effect of Interface (F(5,55)=243.596, p<.001). Pairwise comparisons with Bonferroni correction showed significance across all pairs of interfaces, except for the Phosphor × 125ms, the 125ms × 250ms, and the 250ms × 500ms conditions (Figure 17a and c).

Task time across tasks: We observed a significant main ef- fect of Task (F(2,22)=8.974, p<.001). In pairwise compari- sons using Bonferroni correction, we found that users were significantly slower in the TripleIcon condition (M=1160.4s) as compared to both the Distractor condition (M=995.9, p<.020) and the SingleIcon condition (M=1062.0, p<.019). As one would expect, tracking three icons takes longer that tracking a single icon.

A planned contrast on the Phosphor task times found that all three task conditions were significantly different from each other (F2,22 = 17.68, p < .001). Trials of the TripleIcon task (1010.72ms) took participants 56% longer than the SingleIcon task (645.54ms). The additional two distractor icons of the distractor task increased task time by 25% to 804.25ms per trial for the distractor task.

Task time across outcomes: Finally, we found a significant main effect of Outcome (F(3,33)=966.709, p<.001). Pair- wise comparisons showed all Outcomes significantly differ- ent from each other. The ErrorNeighbor and ErrorOther